Control of plasmid DNA replication in E. coli will be studied using Pl, a bacteriophage whose prophage is a plasmid, as a model system. Mutants which are maintained at multiple copies per cell will be isolated from Pl carrying multiple antibiotic resistance determinants. These mutants will be analyzed, by DNA-DNA hybridization and by genetic techniques to determine the number and location of the genes involved in the control of plasmid copy number. Miniplasmids derived from Pl will be isolated to help define the replication origin and regions involved in regulation of replication. These miniplasmid will then be used as templates for an in vitro replication system which will allow characterization of gene products involved in control of plasmid replication. The generation of a free resistance determinant from PlApCm will also be studied.